1. Field of the Invention
Our invention relates generally to a system for and a method of generating steam for use in enhanced oil recovery processes. More particularly, our invention relates to a system for and a method of producing a steam/water mixture from feedwater having a high total dissolved solids content.
2. Description of the Related Art
Steam injection is used in the oil industry to promote the flow of viscous, heavy oils or liquid hydrocarbons from tar sands. Because the feedwater available to boilers at oil fields is normally of poor quality, having a very high proportion of total dissolved solids (TDS), boilers for such applications usually employ a single-tube flow path throughout the unit. A very high proportion of total dissolved solids (TDS) in this application is intended to mean an amount above about 2,000 ppm, especially, above about 5,000 ppm. The quality of the produced steam, i.e., the ratio of the mass flow rate of the gas phase to the total mass flow rate, is usually limited to not greater than about 80% steam. By maintaining at least this level of residual water throughout the flow path, and by employing a high fluid velocity along the flow path, salts and other dissolved solids are kept in solution to prevent their deposition inside the boiler tubes.
Typical boilers utilized for enhanced oil recovery applications are small-scale, once-through boilers fired with oil or gas. Usually, a single large diameter tube or a few parallel tubes are configured in a helical or serpentine arrangement to form the furnace or combustion chamber enclosure. These tubes then extend into a heat recovery area of an exhaust gas passage to further cool the flue gas and to complete the generation of 80% quality steam.
A natural circulation boiler with a steam drum has also been used for enhanced oil recovery applications. Saturated steam leaving the steam drum is mixed with drum blowdown water to provide 80% quality steam. As steam is generated, the TDS concentration of the water in the boiler increases. With high-TDS feedwater, the tendency for foam formation may become severe, which can cause drum level control problems as well as increased potential for tube failure due to dynamic instability and/or dryout. Therefore, anti-foaming chemicals must be added to the boiler water to minimize foam formation.
For large boiler applications, i.e., when production of more than about 100 tons per hour of the steam/water mixture is required, it is mechanically difficult to design the furnace enclosure to be a once-through configuration. A drum-type boiler simplifies the configuration, but does not eliminate the concerns noted above with respect to high-TDS feedwater.
Our invention provides an improved steam generation system and method suited for use in connection with enhanced oil recovery processes. Our invention is particularly suited for producing a steam/water mixture from water having a high total dissolved solids content, i.e., an amount above about 2,000 ppm, especially, above about 5,000 ppm.
In one aspect, our invention relates to a once-through boiler system for use in conjunction with a combustion chamber. The system includes a water inlet through which water having a high total dissolved solids content is supplied, at least one tubular preheating surface for preheating the water as the water flows through the preheating surface, and at least one tubular evaporation surface for further heating the water flowing therein to produce a steam/water mixture. The preheating surface is disposed downstream from the inlet and encloses at least part of the combustion chamber. Meanwhile, the evaporation surface is disposed within the combustion chamber, downstream from the preheating surface.
Such a boiler system thus differs from conventional systems in that the combustion chamber is enclosed at least in part by one or more preheating surfaces, instead of evaporation surfaces. A benefit of using water to cool the combustion chamber enclosurexe2x80x94as opposed to a steam/water mixturexe2x80x94is that relatively small diameter tubes can be used to form the enclosure, thereby providing more efficient cooling of the enclosure while reducing the likelihood of deposit buildups inside the tubes. In a preferred embodiment, for example, the combustion chamber is enclosed at least in part by a plurality of tubular preheating surfaces, and each of the preheating surfaces comprises a tube panel having a plurality of individual tubes. Preferably, each of the individual tubes has an outer diameter of less than about 50 mm, more preferably less than about 40 mm.
The plurality of preheating surfaces preferably is arranged in a multiple-pass configuration. That is, the preheating surfaces are arranged so that the water makes multiple passes over the combustion chamber enclosure before moving on to the next stage. The multiple-pass configuration permits a relatively high flow velocity to be maintained through the preheating tubes, which further reduces the likelihood of deposit buildups inside the tubes. The multiple-pass configuration also limits the temperature pickup per pass so that temperature unbalances are minimized. Complete mixing between passes further minimizes any unbalances. Preferably, the mass flux of water flowing through the preheating tubes is at least about 1000 kg/m2s, more preferably at least about 1300 kg/m2s.
Meanwhile, the evaporation surface within the combustion chamber preferably comprises a wingwall panel including a plurality of individual tubes. Preferably, each of the individual tubes has an outer diameter of at least about 70 mm, more preferably at least about 90 mm. Preferably, the mass flux of water flowing through the wingwall panel tubes is at least about 1000 kg/m2s, more preferably at least about 1300 kg/m2s.
Preferably, the system further comprises at least one additional tubular preheating surface that encloses at least part of a heat recovery area of an exhaust passage through which exhaust gases are discharged from the combustion chamber. This preheating surface preferably is disposed downstream from the one or more preheating surfaces that enclose at least part of the combustion chamber, but upstream from the one or more evaporation surfaces within the combustion chamber. Preferably, at least part of the heat recovery area is enclosed by a plurality of tubular preheating surfaces that is arranged in a multiple-pass configuration.
Optionally, the system may further comprise at least one more additional tubular preheating surface disposed within the heat recovery area. This preheating surface may comprise, for example, a stringer-type support tube, an economizer, or the like.
Preferably, the system also comprises at least one additional tubular evaporation surface disposed within the heat recovery area, downstream from the evaporation surface within the combustion chamber. In a particularly preferred embodiment, the evaporation surface within the combustion chamber includes an outlet header that is divided into one or more outlet sections, and the evaporation surface within the heat recovery area comprises a corresponding number of individual tubes, each tube being in flow communication with a different one of the outlet sections. Preferably, these individual tubes do not interconnect with each other, thereby reducing the risk of uneven flow distribution through the individual tubes of this evaporation surface.
In another aspect, our invention relates to a method of producing a steam/water mixture from water having a high total dissolved solids content by using a once-through boiler system provided in conjunction with a combustion chamber. The method includes the steps of (i) supplying water having a high total dissolved solids content to the boiler system, (ii) preheating the water by directing the water through at least one tubular preheating surface that encloses at least part of the combustion chamber, and (iii) further heating the water to produce a steam/water mixture by directing the preheated water through at least one tubular evaporation surface disposed within the combustion chamber.
Our invention thus enables the design of a large-scale, once-through boiler that is capable of reliably meeting the requirements for enhanced oil recovery in an efficient and economical way. The concept, however, is also applicable to small size boilers. The invention can be applied to suspension-fired or circulating fluidized bed boilers utilizing a variety of low cost fuels and feedstocks. Compared to conventional boilers having a natural circulation drum-type design, our invention eliminates the need for several pressure components, making our system much more cost effective. Additionally, a boiler system constructed in accordance with our invention is simple, practical, and easy to repair and maintain.